Unintentional falls are common, dangerous, and contribute significantly to mobility impairments in the elderly. Falls to the side often occur and often result in hip fractures, but have seldom been studied. The objective of this proposal is to explore age- related biomechanical declines in the elderly which would impair their ability to restore balance once it has been disturbed. This study will test subjects in groups of 24 healthy young and 24 healthy elderly. The applicant will measure the performance of each subject while they: respond to a large, left or right lateral waist pull; take a voluntary left or right lateral step; and execute a voluntary left or right finger lift task. The responses will be quantified in terms of myoelectric activities and support surface reactions. Premotor latency time, motor latency time, step foot or finger unloading time and step foot movement time will be determined experimentally. The first hypothesis is that the study will find significant age group differences in the biomechanics of the involuntary response to lateral disturbances. The significance of this hypothesis is that the applicants will begin to understand the biomechanical reasons elderly often fall to the side and they plan to investigate a simple clinical test for evaluating the risk of falling. The second hypothesis is that the performance of either upper or lower extremity voluntary tasks are poor predictors of the ability to perform time-critical involuntary tasks. The significance of this hypothesis is that it would suggest replacing the presently used clinical voluntary tasks with time-critical functional tasks in evaluating a person's risk of falling. The third hypothesis is that simple and choice reaction times measured in the lower extremities are significantly longer in the elderly compared to the young. This project should provide important information for the development of clinical evaluation tests used to identify those at risk of falling and the design of effective fall intervention therapies.